1. Field of Invention
The present invention is an improvement over the invention in application Ser. No. 07/791,773 and relates to a low pressure reactive magnetron sputtering apparatus and method for fabricating dielectric optical coatings on substrates. The difference between the invention in this application and the invention in the prior application is the surprisingly high quality of the coatings produced by this invention. The invention in the prior application was concerned with producing dense and stable coatings used for commercial bandpass filters and other types of optical coatings. The invention in this application deals with the specialized field of "low loss" coatings used for laser mirrors and output couplers. In these types of films, film scatter, absorption, and defects must be kept to a minimum. To date, the only known type of film process which has succeeded in this endeavor is Ion Beam Sputtering or IBS. IBS will be further explained hereinafter.
2. Prior Art
Optical coatings requiring very low levels of scatter and absorption have been traditionally manufactured using IBS. With this method, in a very high vacuum environment, a high energy ion beam in the energy range of 500 eV to 1500 eV is directed at a target (source) composed of desired coating materials. The effect of the ion bombardment is to sputter or remove atoms (species or particles) from the target due to momentum exchange in the target lattice. The sputtered species then condenses onto substrates. The pressure in the chamber is desired to be maintained at a very low level to prevent gas-phase collisions of the sputtered particles with background gases.
There is an extensive volume of references to reasons for the improved optical film performance that IBS provides over other coating techniques such as evaporation and other methods of sputtering.
Wei et al initially recognized the benefits of IBS for laser high reflectors in the U.S. Pat. No. Re 32,849 "Method of Fabricating Multi-layer Optical Films". In Wei et al, quarterwave stacks used for laser mirrors are produced using only a single ion gun illuminating a target. FIG. 1, which is a reproduction of FIG. 1 of Wei et al, shows an ion gun "A", target "B" and substrate "C". With this system, background levels of argon (inert gas) was kept at the extremely low pressure level of 1.5.times.10.sup.-4 Torr. Reactive gas pressure (oxygen) was set to a level to insure proper stoichiometry of the depositing levels, in the range of 5.times.10.sup.-5 Torr for high index materials and 3.times.10.sup.-5 for low index materials.
Another IBS patent by Scott et al, No. 4,793,908 "Multiple Ion Source Method and Apparatus for Fabricating Multilayer Optical Films" uses the method of Wei et al with the addition of a second ion beam directed at the substrate which is partly composed of the required reactive species. The second ion beam provides improved optical properties. FIG. 2, which is a reproduction of FIG. 2 of Scott et al, shows ion gun "A", target "B", substrate "C" and the second ion gun "D". In this patent, Scott et al teaches that IBS is improved over conventional magnetron sputtering as the ". . . gas pressure in the chamber, e.g. at the substrate surface, using this approach can be in the tenths or hundreths of a millitorr range. This is a great advantage since the finished film tends to contain fewer gas atoms and have an improved range structure and atomic packing density." Col. 2 lines 14-19.
The films produced by the foregoing inventions have total losses as high reflector laser mirrors well less than 0.01% or 100 ppm.
"Loss" refers to everything other than reflection or Total loss=1-R where R=1-T-A-S and where R is reflection, T is transmission, A is absorption and S is scatter.
Vossen and Kern in the book "Thin Film Processes", Academic Press, New York 1978 at page 189 describe IBS as differing from other sputtering processes due to the fact that "low background pressure gives less gas incorporation and less scattering of sputtered particles on the way to the substrates." ("low background pressure"[sic] are in italics in the original). As discussed previously, this is of great advantage for depositing optical films.
Evaporation techniques, where the coating material is heated under vacuum to the point at which it evaporates, can also be accomplished at low coating pressures comparable to IBS. However, this technique does not impart nearly the kinetic energy of sputtering and the films tend to grow in a porous columnar manner. In addition, the process tends to eject small particles from the hot source due to small source explosions of source materials which can be caused by expanding trapped gases or differential heating. For these reasons, evaporation is used only for the production of relatively low tolerance coatings.
DC or magnetron sputtering has also been used to produce dielectric coatings for many low tolerance applications. In general, these methods involve filling a chamber with inert gas which is then ionized to form a low energy plasma. A target is then charged to a negative potential in the range of 400 to 900 volts which has the effect of bombarding the target with energetic charged ions and sputter atomic or molecular particles from the target. The sputtered particles then condense onto substrates. DC sputtering is used to sputter metals. RF sputtering utilizes oscillating target voltage with a net zero DC current to allow dielectric targets to be sputtered.
In the case of reactive DC sputtering, where reactive gas(es) are added to the chamber to form a compound film at the substrate, it is desired to have a reaction take place on the substrate and not the target, as a severe reduction in deposition rate as well as an increase in target arcing will take place when the target becomes covered with a reactive dielectric species. Many techniques in the prior art exist to cope with this problem, which are all some form of target and substrate isolation, where the reactive gas pressure at the target is maintained at a low level to prevent target "poisoning" and the reactive gas pressure is kept high at the substrate to effect reaction.
In the U.S. Pat. No. to Scobey etal No. 4,851,095 "Magnetron Sputtering Apparatus and Process" parts are shuttled on a high speed drum between a deposition zone maintained at a very high pressure of argon and a reaction zone containing an energetic reactive gas plasma.
The U.S. Pat. No. to Maniv et al 4,392,931 "Reactive Deposition Method and Apparatus" target material is sputtered through an orifice or aperture onto a rotating drum. Inert working gas is bled into the target chamber, and a reactive gas is bled into the rest of the chamber. The aperture limits the amount of reactive gas to the target. A field is established on the drum to ionize reactive gas and increase film transparency.
Scherer et al U.S. Pat. No. 4,931,169 "Apparatus for Coating A Substrate with Dielectrics" also discloses a sputtering through an orifice and producing an AC component to the DC drive voltage to prevent arcing. The AC field has the added effect of increasing rate due to an increase in collisions between oscillating electrons and the working gas. The field has the further effect of allowing a reduction in the coating pressure to as low as 0.5 Torr.
The U.S. Pat. No. to Dietrich et al 4,946,576 "Apparatus for the Application of Thin Layers to a Substrate" also discloses the use of an aperture between the cathode and the substrate and adds a positive voltage near the substrate over which the reactive gas flows. The reactive gas becomes ionized by the anode which has the effect of improving film stoichiometry. Another U.S. Pat. No. to Dietrich et al, 4,572,840 "Method and Apparatus for Reactive Vapor Deposition of Compounds of Metal and Semi-Conductors" uses a flow restriction between the magnetron and substrate equal to at least 40% of the cross-section of the space.
In all of the above cited prior art, the source to substrate distance is short. In Scobey et al, the distance is approximately 10 cm; in Maniv et al, the distance is 10 cm; in Scherer et al, the distance as 4 cm and Dietrich et al, '842, uses an example of 6 cm while Dietrich et al '576, does not list a distance but from the drawings it appears to be about 10 cm.
In addition, in all of the above cited prior art, the total pressure is maintained at conventional sputtering pressures of approximately 3.times.10.sup.-3 Torr between the substrate and target, except for the Scherer et al.
Also, none of the magnetron sputtering systems cited above were capable of producing, or claimed the ability to produce, low loss optical coatings.